With adhesive products, high performance and rigidity are often thought to go hand in hand. And it is true that the very best strength, thermal, chemical and electrical properties tend to be found in rigid compounds, especially epoxies.
Yet there is a growing class of adhesives, sealants and coatings that add ductility to the long list of desirable epoxy properties.
These "toughened" epoxy compounds can be created in a variety of ways. Toughening sometimes involves the incorporation of ductile rubber or thermoplastic materials into epoxy's normally rigid molecular backbone. These ductile materials may remain in the epoxy compound as distinct phases, absorbing shear or impact forces.
Carboxylterminated butadiene-acrylonitrile (CTBN) copolymers are one family of ductile materials that Master Bond has used to great advantage in a variety of toughened compounds.
Other toughening methods rely on specific diluents or other reactive elements to change the epoxy's molecular structure in ways that make it more flexible - for example, by reducing the crosslink density or by shortening molecular chains. Heat cure schedules can also be manipulated to improve epoxy ductility.
Every toughening method has its advantages and disadvantages, but they all address the chief shortcoming of epoxy compounds.
For all their first-rate mechanical, thermal and chemical properties, conventional epoxies can sometimes be too rigid for use in structural applications subject to extremely heavy fatigue, impact or thermal shock loads.
Toughened epoxies have no such Achilles Heel. They can be used in scores of electrical, optical and medical applications where rigidity might otherwise prohibit epoxy usage.
When comparing a toughened and conventional epoxy, the mechanical property difference that jumps off the data sheets is elongation.
Whereas a traditional epoxy will have elongations of less than 5%, toughened epoxies typically have elongations that fall within the range of 50 to 80%.
This elongation improvement represents a huge increase in ductility, which allows the adhesive to withstand impacts and "flex" under cyclic loads caused by fatigue or temperature swings.
Lastly, toughened compounds tend to cure with lower residual stresses than their more rigid counterparts. Unlike conventional expoxies, they often have residual stresses approaching zero.
Ultra-low-stress curing is particularly important when bonding or encapsulating delicate electronic, optical or medical components. These components are often made from materials whose yield stresses are far below the residual stresses possible with standard epoxies, making them susceptible to damage. In optical applications, residual stresses can cause additional problems by misaligning components or inducing birefringence.
With toughened compounds, ductility does come with a small trade-off related to maximum service temperature.
While high temperature epoxies can withstand intermittent temperatures above 300°C, toughened products top out at about 200°C intermittent. Reductions in typical continuous use temperatures are similarly about 100°C.
This loss of thermal properties, though seemingly significant, does not have widespread practical implications since the vast majority of bonding, potting and coating applications do not require anywhere near the maximum temperature.
At very low temperatures, which tend to magnify any ductility issues an adhesive might have, toughening actually imparts a performance advantage. Nearly all cryogenically serviceable epoxies, some of which have to withstand temperatures as low as 4K, have some sort of toughening agent.
Other than the differences in thermal properties, toughened epoxy compounds do not differ significantly from their unmodified counterparts.
To take three key property examples, the chemical resistance, electrical properties, and bond strength are all comparable.
Finally, toughened adhesives offer excellent adhesion to many metal, plastic, glass and rubber substrates.
And in the case of rubber and elastomeric substrates, toughened compounds are often a necessity. Putting a rigid compound between flexible substrates is a recipe for failure.
Thanks to their ability to accommodate challenging load cases, toughened epoxies are gaining ground in a wide variety of applications.
Many of them involve the bonding, sealing or potting of electronic and optical components, which can be sensitive to the very issues that toughened compounds address: impact, vibration, thermal cycling and stresses.
Toughened compounds are also available in versions that offer speciality properties, which opens up applications that require ductility in conjunction with other functional attributes.